This type of turboshaft engine is fitted with a regulator system whose main function is to regulate the power delivered by the engine during flight by controlling the fuel flow rate. The speed of rotation of the main rotor for providing the rotorcraft with drive and lift is then maintained at a value that is substantially constant.
A rotorcraft is controlled in particular by acting on the pitch of blades of said main rotor. An increase in pitch leads to the speed of rotation of the rotor falling off quickly. The engine must then accelerate quickly so as to compensate for this drop in speed in order to enable the rotorcraft to be maintained in flight. Similarly, when the blade pitch is reduced, it is necessary to decelerate the engine so that the speed of rotation of the rotor does not exceed a limit determined by the manufacturer.
In addition, the fuel flow rate must be regulated scrupulously in such a manner as to ensure that the main rotor can absorb an increase in power without running the risk of engine pumping. Pumping is a phenomenon that affects the compressors of turboshaft engines when the angle of incidence of a rotor blade or of a rectifier becomes locally too great, leading to aerodynamic separation (stalling) which considerably reduces the air flow rate. A consequence of this phenomenon is overheating in the combustion chamber which can lead to deterioration of the turbine of the engine.
Similarly, deceleration must also be controlled so as to avoid engine flameout.
An electronic regulator apparatus is known to the person skilled in the art under the name FADEC (full authority digital engine control). Regulation relationships, e.g. for acceleration or deceleration, are programmed in the FADEC so as to enable it to regulate the fuel flow rate without running any risk of pumping or of flameout for the engine.
In addition, the FADEC receives signals from sensors that measure various parameters of the engine, in particular pressure at the outlet from the compressor stages, the speed of the engine's gas generator, the speed of the free turbine, and the internal temperature at the inlet to the free turbine, which parameters are respectively written P3, Ng, NTL, and T4 by the person skilled in the art. As a function of this information, the FADEC makes use of regulation relationships for controlling an actuator that adjusts the fuel flow rate by acting on the setting of the engine's fuel metering system.
That apparatus gives satisfaction, but it is not sufficiently reliable on its own to guarantee safe flight. Backup systems therefore need to be installed on the rotorcraft in order to remedy this drawback.
The regulation relationship used for rotorcraft turboshaft engines depend on the quotient of the fuel flow rate divided by the pressure P3. The justification of this principle lies in its stabilizing nature since, in the event of pumping, the pressure P3 drops, and thus causes the fuel flow rate to be lowered, which often makes it possible to escape from the phenomenon. Unfortunately, experience shows that the sensor for sensing the pressure P3 is particularly fragile and is at the root of many anomalies.